1. Field of the Invention
The present invention relates to a steel material that has high corrosion resistance and is suitable for a fabric which is difficult to maintain and is used with or without painting, for example, a bridge; and a fabric using this steel material.
2. Related Art
Hitherto, a steel material used for bridge fabrics, for example, a traffic bridge in the environment which may easily be subjected to corrosion (which may be referred to chloride environment, hereinafter) by airborne salt from the sea or deicing salt, such as an area in mountains or a seaside area, has been painted and used to improve corrosion resistance. However, the painted coating necessarily deteriorates with the passage of time. Thus, in order to maintain corrosion resistance, it is necessary to repaint the steel material in a given cycle.
Recently, as such a bridge, there has been frequently used a small number main girder bridge, which has a few main girders and includes a 2-main girder bridge as a typical example, instead of a conventional large number main girder bridge. The small number main girder bridge has the following advantages over the large number main girder bridge: the amount of used steel materials (steel weight) and the number of bridge members can be reduced and further construction efficency is good, so as to contribute to environmental protection and shortening of a construction term. For such a small number main girder bridge, it has been demanded that costs and loads for maintenance after the construction of the bridge are reduced as much as possible and the life span of the bridge itself is prolonged.
Therefore, for steel materials used for fabrics such as steel towers and buildings including the above-mentioned small number main girder bridge, it has been demanded that high corrosion resistance is maintained. For example, it has been demanded that the maintenance after the construction of the bridge is unnecessary even if the steel materials are used without any painting in the environment which may easily be subjected to chlorides, or the steel materials are used with painting and the painted coating is deteriorated or damaged in the use.
In order to improve corrosion resistance of such steel materials, improving-techniques about its parent metal, i.e., steel have been hitherto proposed. A typical example thereof is a weathering steel comprising P: 0.15% or less, Cu: 0.2-0.6%, Cr: 0.3-1.25%, and Ni: 0.65% or less. As this weathering steel, the following 2 types are standardized: types according to JIS G 3114 (Hot-Rolled Atmospheric Corrosion Resistant Steels for Weld Use) and JIS G 3125 (Superior Atmospheric Corrosion Resistant Rolled Steel). This weathering steel has such a self corrosion resisting function that rust generated on the surface of the steel in use of the steel becomes protective and this protective rust layers (weathering rust) having high atomospheric corrosion resistance by the above-mentioned elements in very small amounts. By such a property, the weathering steel is used mostly without any painting, as a maintenance-free construction material for various fabrics, for example, the above-mentioned bridge.
In the chloride environment, however, the protective rust layers, which characterize the weathering steel, are not easily formed by the influence of chloride. If the protective rust layers are not formed, the corrosion resistance of the weathering steel is remarkably lowered. This is based on the fact that the corrosion of the steel results in a drop in pH inside the rust layers in the chloride environment. That is, even if corrosion of the steel occurs slightly, the pH at the surface of the steel is lowered by the reactions of Fe.fwdarw.Fe.sup.2+ +2e.sup.- followed by Fe.sup.2+ +2H.sub.2 O.fwdarw.Fe(OH).sub.2 +2H.sup.+ so that both pH inside the rust layers and pH at the interface between the rust layers and the steel are lowered. Once these pHs are lowered, the transport number of chlorine ions in the rust layers increases to keep electrical neutralization. Thus, the concentration of chlorine ions occurs at the interface between the rust layers and the steel. As a result, a hydrochloric acid atmosphere is produced around the interface to promote the corrosion of the steel. At the same time, the solubility of iron ions increases by the drop in pH inside the rust layers to result in the phenomenon of blocking the formation of the protective rust layers, which is a main point of corrosion resistance function of low alloy corrosion resistant steels such as weathering steel. Thus, the corrosion is accelerated.
Therefore, in order to prevent the drop in pH inside the rust layers, there is proposed a technique of making the surface of the weathering steel alkali to block the acceleration of the corrosion. More specifically, Japanese Patent Application Laid-Open (JP-A) No. 58-25458, Japanese Patent No. 2572447 and the like disclose a method of dispersing an oxide (chemical species)of Be, Mg, Ca, Sr, Ba or the like for making the surface of weathering steel alkaline into the steel beforehand and acting the chemical species at the same time of the corrosion reaction of the steel to suppress the drop in pH at the steel surface.
The method of adding such an oxide to block acceleration of corrosion is indeed advantageous from the standpoint of suppression of the influence of chlorides from the external environment. However, it is difficult or restrictive in the same way as in the above-mentioned weathering steel to form the protective rust layers themselves. As a result, sufficient corrosion resistance cannot be obtained under the actual conditions. It is also feared that the oxide added to the steel has a bad influence on weldability and/or strength.
For this reason, in order to improve the corrosion resistance of steel materials, there are proposed various treatment processes for promoting protective rust formation by subjecting the steel materials to surface treatment, instead of the above-mentioned method of adjusting the components or composition of steel materials. For example, JP-A-6-93467 discloses a method of covering the surface of steel with rust comprising .alpha.-FeOOH containing 0.3 wt % or more of one or more elements selected from Cr, Cu, P and Ni, and painting an aqueous solution containing Cr, Cu, P or Ni onto the surface of the steel in order to form this rust.
JP-A-9-125224 discloses a method of covering the surface of steel with rust comprising hematite (.alpha.-Fe.sub.2 O.sub.3) of 30-200 .mu.m in thickness by heat-treating the steel.
The methods in the prior art of forming the protective rust layers themselves by surface treatment or heat treatment of steel materials are notable methods from the standpoint that attention is paid to the components or the composition of the protective rust layers. Namely, the above-mentioned weathering steel or the oxide-dispersed steel contains alloying elements in large amounts so as to exhibit inevitably lower weldability at the time of processing the steel material or lower efficiency at the time of producing the steel material by melting, rolling or the like than that of ordinary steel. Costs for producing the steel material are also high by the drop in production efficiency and the inclusion of relatively large amounts of the alloying elements. Besides, the drop in the weldability makes costs for construction using the steel material high. Accordingly, if there is a method wherein ordinary carbon steel or low alloy steel is used without using such a weathering steel to realize high corrosion resistance by adjusting the structure and/or the composition of the protective rust layers, this method has many advantages from the viewpoint of construction efficency and costs.
However, the inventors have found that when the rust comprising .alpha.-FeOOH containing 0.3 wt % of one or more selected from Cr, Cu, P and Ni, as disclosed in JP-A-6-93467, or the rust comprising hematite (.alpha.-Fe.sub.2 O.sub.3) as disclosed in JP-A-9-125224 is used without any painting in the chloride environment, or is used with painting and the painted coating is deteriorated or damaged, high corrosion resistance is not necessarily exhibited. The inventors have also found that this is caused by the facts that the rust layers cannot easily be produced and that even if chemical treatment or heat treatment is conducted for the production thereof, the reproducibility of the rust layers is not good.
Therefore, an object of the present invention is to provide a steel material that has high reproducibility and corrosion resistance and is suitable for fabrics used with or without painting even if the steel material belongs to ordinary carbon steel or low alloy steel.